Co-current and counter-current configurations for ethanol steam reforming in a dense Pd–Ag membrane reactor F. Gallucci a , M. De Falco b , S. Tosti c , L. Marrelli b , A. Basile d, * a Fundamentals of Chemical Reaction Engineering Group, Faculty of Science and Technology, University of Twente, Enschede, The Netherlands b Universita ` di Roma La Sapienza, Dip. Ing. Chimica e Materiali, Via Eudossiana 18, 00184 Roma, Italy c ENEA,Dipartimento Fusione, Tecnologie e Presidio Nucleare, C.R. ENEA Frascati, Via E. Fermi 45, 00044 Frascati (RM), Italy d Institute on Membrane technology, ITM-CNR, c/o University of Calabria, Via Pietro Bucci, Cubo 17C, 87030 Rende (CS), Italy article info Article history: Received 8 April 2008 Received in revised form 3 July 2008 Accepted 3 July 2008 Available online 25 September 2008 Keywords: Ethanol steam reforming Membrane reactor Pd membranes Hydrogen production Modelling abstract The ethanol steam-reforming reaction to produce pure hydrogen has been studied theo- retically. A mathematical model has been formulated for a traditional system and a palladium membrane reactor packed with a Co-based catalyst and the simulation results related to the membrane reactor for both co-current and counter-current modes are pre- sented in terms of ethanol conversion and molar fraction versus temperature, pressure, the molar feed flow rate ratio and axial co-ordinate. Although the counter-current mode does not always give an ethanol conversion higher than the one obtained in membrane reactor operated in co-current mode, in the first case it is always possible to extract more hydrogen from the reaction zone. With this theoretical analysis, different values of the operating parameters that allow to have a CO-free hydrogen stream and a complete recovery of the hydrogen from the lumen side of the reactor are investigated. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. 1. Introduction Polymeric fuel cell systems are electrochemical devices able to generate electrical power by the electrochemical oxidation of hydrogen with atmospheric oxygen. Many studies about hydrogen production for fuel cells deal with the use of two types of carbon compounds: the first is an oxygen-containing compound, such as methanol or ethanol, while the second is hydrocarbons, such as natural gas, propane gas, gasoline, etc. [1–8]. Production of hydrogen from ethanol is very attractive since bio-ethanol is a renewable material mainly produced from biomass fermentation [9,10]. Many authors studied the ethanol steam reforming [11–17] being this an endothermic catalysed reaction whose conver- sion increases with the temperature. Usually, catalysts based on Rh, Ru, Pd, Pt, Ni, Co and Cu are used on supports of Al 2 O 3 , SiO 2 , MgO and La 2 O 3 . The reaction conversion and selectivity of the products (mainly H 2 , CO, CO 2 and in minor part, CH 4 , CH 3 CHO and C 2 H 4 ) depend on the catalyst used as well as on the operating temperature. The ethanol steam reforming was studied in innovative reactors such as membrane reactors and microreactors [18]. The use of membrane reactors for carrying out the steam reforming is proposed in order to increase the ethanol conversion at lower temperature. In fact, a membrane reactor is a device in which a reaction and a selective separation takes place simultaneously: in this way, the continuous removal of one of the product permits obtaining reaction conversion beyond the thermodynamic equilibrium that is an upper limit to be considered in a traditional reactor (shift effect) [19–21]. * Corresponding author. Tel.: þ39 0984 492013; fax: þ39 0984 402103. E-mail address: [email protected](A. Basile). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.07.026 international journal of hydrogen energy 33 (2008) 6165–6171
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Co-current and counter-current configurations for ethanol steam reforming in a dense Pd–Ag membrane reactor
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 6 1 6 5 – 6 1 7 1
Avai lab le a t www.sc iencedi rec t .com
j ourna l homepage : www.e lsev ier . com/ loca te /he
Co-current and counter-current configurations for ethanolsteam reforming in a dense Pd–Ag membrane reactor
F. Galluccia, M. De Falcob, S. Tostic, L. Marrellib, A. Basiled,*aFundamentals of Chemical Reaction Engineering Group, Faculty of Science and Technology, University of Twente, Enschede,
The NetherlandsbUniversita di Roma La Sapienza, Dip. Ing. Chimica e Materiali, Via Eudossiana 18, 00184 Roma, ItalycENEA,Dipartimento Fusione, Tecnologie e Presidio Nucleare, C.R. ENEA Frascati, Via E. Fermi 45, 00044 Frascati (RM), ItalydInstitute on Membrane technology, ITM-CNR, c/o University of Calabria, Via Pietro Bucci, Cubo 17C, 87030 Rende (CS), Italy
best result (94%) is obtained only at high pressure and high
temperature.
3.4. The effect of the sweep gas flow rate
The effect of the sweep gas flow rate on both the ethanol
conversion and the hydrogen recovery is well illustrated on
Fig. 8. By increasing the sweep gas molar ratio, calculated as
the ratio between the sweep gas flow rate and the ethanol feed
flow rate, the ethanol conversion in both co-current and
counter-current mode increases. In particular, for sweep ratio
higher than 2 the counter-current mode gives higher conver-
sion than the co-current mode. This fact indicates that the
domain where the co-current mode gives better results than
the counter-current one depends on the pressure and the
temperature (as already indicated in Fig. 6), but also from the
sweep gas flow rate. Moreover, for sweep ratio higher than 10,
the counter-current mode is able to give complete ethanol
conversion while this value is never reached in the co-current
mode in the whole range of sweep ratio investigated. Con-
cerning the hydrogen recovery, the same Fig. 8 clearly indi-
cates that the counter-current mode gives complete recovery
and reaction conversion for sweep ratios higher than 5, while
the highest hydrogen recovery obtained in the co-current
mode is reached at sweep ratio¼ 10 and it is 95%. In particular,
this value is a plateau for the co-current mode operated at the
conditions illustrated in Fig. 8.
3.5. The combined effect of pressure and feed molar ratio
The three dimensional Figs. 9 and 10 report for both co-current
and counter-current modes the effect of pressure and water/
ethanol feed molar ratio on the ethanol conversion and
hydrogen recovery, respectively. In particular, Fig. 9 shows that
for both co-current and counter-current modes the ethanol
conversion sharply increases for feed molar ratio between 2
and 3, for each value of pressure investigated, while it slightly
increases for feed molar ratios higher than 3. Moreover, for
pressures lower than 3 bar and feed molar ratio higher than 3,
Sweep ratio, -
0 5 10 15 20 25 30
Eth
an
ol co
nversio
n, %
75
80
85
90
95
100 Counter-current
Co-current
Sweep ratio, -
0 5 10 15 20 25
Hyd
ro
gen
reco
very, %
75
80
85
90
95
100
Co-current
Counter-current
Fig. 8 – Ethanol conversion and hydrogen recovery versus
sweep gas/ethanol ratio. Co-current and counter-current
modes, water/ethanol [ 3, T [ 673 K, plumen [ 3 bar,
pshell [ 1 bar.
50
60
70
80
90
100
2 3 4 5 6 7 8 9 10 11
23
45
67
Eth
an
ol co
nverstio
n, %
Water/Ethanol feed ratio, -
Pressure, b
ar
Counter-current
Co-current
Fig. 9 – Ethanol conversion versus pressures and water/
ethanol feed ratio. Co-current and counter-current modes,
sweep gas/ethanol [ 2, T [ 673 K, pshell l[ 1 bar.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 6 1 6 5 – 6 1 7 16170
the co-current mode is able to give better results in terms of
ethanol conversion than the counter-current one does.
Considering the effect of the molar feed ratio on the
hydrogen recovery, it is evident in Fig. 10 that the hydrogen
recovery in counter-current mode is not influenced by the
feed molar ratio while the co-current mode is strongly influ-
enced by this parameter. In fact, in co-current mode, for the
whole range of pressure investigated, the hydrogen recovery
decreases by increasing the feed molar ratio, even though this
decrease is lower at higher pressures.
On the one hand, by increasing the feed molar ratio part of
the excess of water dilutes the hydrogen produced in the
50
60
70
80
90
100
23
45
67
89
1011
23
45
67
Hyd
ro
gen
reco
very, %
Counter-current
Co-current
Wate
r/Eth
anol feed ra
tio, -
Pressure, b
ar
Fig. 10 – Hydrogen recovery versus pressures and water/
ethanol feed ratio. Co-current and counter-current modes,
sweep gas/ethanol [ 2, T [ 673 K, pshell [ 1 bar.
reaction zone resulting in a lower hydrogen partial pressure
difference between the lumen side and the shell side of the
reactor and so in a lower hydrogen recovery. On the other
hand, from Fig. 10 it seems that the counter-current mode
does not suffer by this aspect and this fact could be related to
the different partial pressure profiles between the two modes.
Therefore, it is also worth noting that, at low pressures the
counter-current mode is able to give an increase at least of
30% on the hydrogen recovery with respect to the co-current
mode for each feed molar ratio considered, while at high
pressure the difference between the co-current mode and the
counter-current one decreases up to 5% at 8 bar and water/
ethanol¼ 2.
4. Conclusions
The ethanol steam-reforming reaction to produce pure
hydrogen has been studied theoretically in a Pd-based
membrane reactor for both co-current and counter-current
modes. The results indicate that the counter-current mode is
able to give higher hydrogen recovery than the co-current
mode in the whole range of operative conditions investigated.
For what concerns the ethanol conversion, this theoretical
study also indicates that attentions should be paid in order to
identify the set of parameters able to maximise it in the
counter-current mode. In fact, low pressures and low
temperatures give higher ethanol conversion in the co-current
mode while high pressures and high temperatures give higher
ethanol conversion in the counter-current mode.
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